1. Field of the Invention
The present invention relates to a light source apparatus for an endoscope which is designed so that it is possible to control the luminous flux of illuminating light that is supplied from a light source to the entrance end of a light guide.
2. Description of the Prior Art
A conventional endoscope light source apparatus of the type described above is generally arranged such that illuminating light emitted from a light source lamp is formed into a bundle of parallel rays by a reflecting mirror, and the parallel rays are focused by a condenser lens to enter the entrance end of an illuminating light guide.
In addition, a movable stop is provided in the intermediate portion of an illuminating optical path extending from the light source lamp to the entrance end of the light guide to control the luminous flux of illuminating light entering the light guide by blocking a desired cross-sectional area of the bundle of illuminating light rays from the outer edge thereof.
The conventional light source apparatus suffers, however, from the problem that the control of the incident luminous flux causes a change in the course of passage of illuminating light entering the light guide, resulting in considerable variation in the light-distribution and spectral characteristics of illuminating light emanating from the light guide. To overcome such a problem, it is necessary to use a complicated stop configuration and a complicated driving mechanism for the stop, which results in increase in the production cost.
To solve the above-described problems, Japanese Patent Application Laid-Open (KOKAI) No. 1-200217 proposes an arrangement in which at least a pair of light-blocking plates, which are formed in the shape of plane-parallel plates, are disposed between the condenser lens and the light source such that the light-blocking plates symmetrically face each other across the optical axis of the bundle of illuminating light rays, and in which the direction of the light-blocking plates is changed by rotating them about an axis which extends perpendicularly to the optical axis of the bundle of illuminating light rays and in parallel to the light-blocking plates, thereby controlling the luminous flux of illuminating light entering the light guide. With the proposed arrangement, it is possible to minimize variation in the light-distribution and spectral characteristics of illuminating light emanating from the light guide with a simple structure.
However, it is still impossible to avoid a change of the course of passage of illuminating light caused by a change in the luminous flux of illuminating light entering the light guide. Therefore, when the luminous flux of illuminating light entering the light guide is changed to a considerable extent, a distinct change occurs in the light-distribution and spectral characteristics of light emanating from the light guide, giving rise to a problem in practical use.
When a xenon lamp, which is bright and generates a relatively small quantity of heat, is used as a light source lamp, flicker is likely to occur in the central portion of the bundle of light rays emitted from the lamp because of an electric discharge between the electrodes or the convection of the xenon gas.
Such flicker is not so conspicuous in view of the whole illuminating light. However, as the cross-sectional area of the bundle of illuminating light rays is blocked from the marginal portion thereof, the proportion of central rays to the marginal rays increases and hence flicker becomes even more conspicuous, hindering to the endoscopic observation.